The present invention relates to a process for fabricating a semiconductor device, and particularly to a process for forming a monocrystalline semiconductor film on an insulator.
To meet the demand for high-speed and high-packaging-density semiconductor devices, attempts are being made to produce integrated circuits of smaller stray capacitance wherein the circuit elements are isolated from each other by dielectrics. According to one attempt, the circuit elements are formed within an island of semiconductor crystal on an insulator. A typical method of forming a single semiconductor crystal consists of first depositing a polycrystalline or amorphous semiconductor film on an insulator, and then converting the surface of the film to a monocrystalline semiconducter layer by heating it with high-energy radiation such as a laser or electron beam.
This conventional method is described by reference to FIG. 1. In FIG. 1(a), the reference numeral 10 indicates a quartz (SiO.sub.2) substrate, and 11 is a layer of polycrystalline silicon formed to a thickness of 5,000 .ANG. on the substrate by a conventional chemical vapor deposition method (CVD) in reduced pressure. As shown in FIG. 1(b), an oxide film 12 is formed to a thickness of 500 .ANG. on the silicon layer at 950.degree. C. in an oxidizing atmosphere, and then a nitride film 13 is formed to a thickness of 1,000 .ANG. on the oxide film by CVD under a reduced pressure, and the nitride film is photoetched to a predetermined pattern as shown in FIG. 1(c). The product is exposed to an oxidizing atmosphere at 950.degree. C. for an extended period until all areas not covered with the nitride film 13 are oxidized, and thereafter the nitride film and the underlying oxide film 13 are removed to provide a structure which, as shown in FIG. 1(d), has a layer of polycrystalline silicon surrounded by insulating silicon dioxide (quartz glass) on the bottom and four sides. But the polycrystalline silicon as such does not have crystallinity suitable for fabrication of the desired semiconductor device, so it must be converted to monocrystalline silicon or recrystallized to polycrystalline silicon of a large grain size by fusing it with high-energy radiation such as a narrow laser beam or electron beam. However, in the conventional method, heat dissipates in an uncontrolled manner through the surrounding insulation layer downwardly and transversely, so premature cooling often occurs and many crystal nuclei form to make the production of large grains difficult, to say nothing of the formation of single crystals. To avoid this problem, an insulating layer, for instance a nitride film 15 having a thickness of about 100 .ANG., may be formed on the whole surface of the polycrystalline silicon layer 11 and silicon dioxide layer 14 as shown in FIG. 1(e), and by so doing, heat conduction through the polycrystalline silicon can be controlled in such a manner that it is converted to a layer of monocrystalline silicon. But this technique is not applicable to a case where the heat source is high-energy radiation other than a laser beam. Even if a laser beam is used, the nitride film 15 serves as an anti-reflection layer, and the slightest change in its thickness may cause fluctuations in the laser output, which in turn leads to the formation of a monocrystalline silicon layer whose thickness varies from operation to operation.
FIG. 2 shows the sequence of one method of forming an MOS transistor from an island of the so prepared monocrystalline silicon layer. The process of forming a silicon-gated MOS transistor is well known and requires no detailed accounting. But the major problem with this process arises in the gate oxide film which is indicated at 21 in FIG. 2(b). Since the island silicon layer does not have a controlled grain size or crystal axis, the oxide film formed on it does not have a uniform thickness, nor can it have a uniform distribution of charge. If MOS transistors are formed using this oxide film, their characteristics will vary within the same wafer or from one wafer to another. Turning now to FIG. 2(c), a polycrystalline silicon gate 22 is formed on the oxide film 21 by photo-etching and in FIG. 2(d), a source-drain region 23 is formed under the oxide film 21, and as shown in FIG. 2(e), a complete device is produced by forming layer-isolating insulating films 24, an aluminum wire connection 25 and a surface protective film 26.